Small satellites (also known as PicoSats, NanoSats, or CubeSats) have low weights and small sizes, to reduce launch cost to orbit. A CubeSat, for example, weighs less than a kilogram, occupies a volume of about 1 liter, and has a limited amount of available room for auxiliary systems, such as a flight direction sensor system for efficient orbital positioning. For example, thrusters associated with the satellite need to be coordinated with the direction of travel (or the so-called ram direction) to apply thrust either in the ram direction (for decreasing the orbit radius) or in the anti-ram direction (to increase the orbit radius).
Previous approaches to flight direction sensing have involved calculation of the satellite attitude via combinations of sun, star, Earth horizon, and other sensors. Sun sensors, for example, can measure the angular position of the sun along two orthogonal directions with respect to the spacecraft body. Star sensors may calculate the angles between visible stars, and may search a database of known star positions to determine the spacecraft orientation in inertial space. Such approaches often require multiple sensors and complex imaging systems that can be prohibitively bulky.
The atmosphere rotates with the planet, so spacecraft in low-earth-orbit fly through this atmosphere at 7 to 8 km/s. Pressure sensing approaches to determining flight direction include direct physical sensing of the pressure difference between leading and trailing edges of the spacecraft, or monitoring neutral wind direction. These approaches work best at low (less than 500-km) altitudes where the atmospheric density is readily detectable. The atmospheric density drops rapidly with increased altitude, and therefore, detecting flight direction using pressure sensing becomes almost impossible at altitudes greater than 1,000 km.